1. Field of the Invention
The present invention relates to a diesel engine control system that can perform a specific injection type, in which latter stage injection is ignited by combustion of former stage injection, and the latter stage injection is not ignited upon extinguishment of the former stage injection.
2. Description of Related Art
As one type of multi-stage injection operation, there has been proposed a retarded combustion operation. In the proposed retarded combustion operation, fuel injected into a cylinder by pilot flame generative injection (a former stage injection in an injection pattern of the retarded combustion operation) from an injector is compressed and is ignited, and large heat generative injection (a latter stage injection in the injection pattern of the retarded combustion operation) is carried out at a retarded angle where sole combustion of the large heat generative injection is difficult, so that the large heat generative injection is ignited by the pilot flame combustion caused by the pilot flame generative injection. In this way, the combustion angle of the main combustion (the combustion caused by the large heat generative injection) is retarded, and thereby the undesirable components of the exhaust gas, particularly, NOx is reduced in the retarded combustion operation (low NOx operation).
As discussed above, the main combustion is performed in the respective cylinder, in which the internal volume is increased, and therefore the pressure and the temperature are reduced, so that the NOx in the exhaust gas is reduced (the amount of NOx is increased as the temperature and pressure are increased).
As another technique for reducing the combustion temperature, there is also known an exhaust gas recirculation (EGR).
In the case of executing the retarded combustion operation, it is required to make sure that the pilot flame generative injection is effectively performed, and the fuel of the pilot flame generative injection is effectively combusted.
This is due to the fact that when the combustion of fuel of the pilot flame generative injection is extinguished, fuel injected by the large heat generative injection at the latter stage cannot be ignited. This will likely cause a decrease in the engine torque and/or the engine rotational speed, so that the uncombusted gas may be disadvantageously exhausted from the engine.
The devices of the fuel injection system, such as injectors, may change its output characteristic due to, for example, a change with time (aging), so that the injection quantity of the injector will be possibly changed.
Thus, the combustion of fuel of the pilot flame generative injection could be extinguished due to, for example, the change with time in the devices of the fuel injection system, such as the injectors. This limits use of the retarded combustion operation, in which the above NOx reducing technique is implemented.
Furthermore, in order to more reliably perform the pilot flame combustion, it is possible to increase the pilot flame generative injection quantity (the former stage injection quantity in the injection pattern of the retarded combustion operation). However, this will cause a pressure increase and a temperature increase due to the pilot flame combustion. This will, in turn, cause deterioration of the NOx reducing effect at the time of combusting the large heat generative injection. As a result, it is not desirable to excessively increase the pilot flame generative injection quantity.